Renting rigs can be extremely profitable, and in my view, less risky than buying into newer coins with less than stable prices. You can always calculate the expected profitability of mining in terms of btc/mh/day if you know the block reward, difficulty, and price. Just estimate conservatively and see if your profit calculation beats the rental price. If it does, you'll probably make money. Even if you don't earn back the full rental price, you'll only lose a percentage of the rental price, which is likely to be much less than you would have lost with trading. Note that you can rent for as little as 3 hours, which is what I tend to do when renting to mine newer coins.
Basically: I think the potential upside to renting is slightly less than trading, but the downside potential is dramatically less.
Shhhh.. we don't want everybody stealing the cheap rigs!
I agree, renting can be profitable, though it's usually only feasible to rent when a coin is in its infancy. Once a coin starts getting hammered by miners the btc/mhash return goes way down.. unless it's an uncommon case as with SPA/AUR/etc.
